The ongoing goal of my research program is to understand the neurobiology of early olfactory learning. To date, we have described a series of metabolic, morphological, neurochemical and neurophysiological changes in the olfactory bulb of young rats as a consequence of early olfactory learning and have also shown that an analogous form of learning exists in human infants. In this resubmission, I propose to investigate the mechanisms underlying the critical role of noradrenaline in this type of developmental plasticity. Given the importance of this neurotransmitter in other forms of brain development, we hope to illuminate what may be general mechanisms for brain organization and plasticity. We propose to determine: 1. The response of olfactory bulb neurons to noradrenergic stimulation. 2. Whether there is a transient expression of beta noradrenergic receptors in the developing bulb. 3. Whether blocking alpha noradrenergic receptors will interfere with the expression of the neurobehavioral response after early olfactory preference training. 4. The function of the conditioned increase in bulb noradrenaline. 5. Whether the end of the sensitive period for olfactory preference formation is due to the diminution of the noradrenergic response and/or a change in the response of the bulb to noradrenergic stimulation. 6. Whether noradrenaline is a common critical factor for other reinforcing stimulation during early olfactory learning. 7. Whether "overstimulation" blocking of early olfactory learning is due to very high levels of noradrenaline or a suppression of noradrenaline levels with high levels of stimulation.